docs: def base_url(self):
          '''The public URL for this storage'''
          config_value = self.config.get('url')
          if config_value:
              return self._clean_url(config_value)
          default_url = current_app.config.get('FS_URL')
          default_url = current_app.config.get('{0}URL'.format(self.backend_prefix), default_url)
          if default_url:
              url = urljoin(default_url, self.name)
              return self._clean_url(url)
          return url_for('fs.get_file', fs=self.name, filename='', _external=True)